This invention relates to low dielectric constant layers for use in various applications. The invention also relates to methods of forming low dielectric constant layers and integrating them into a wide range of VLSI fabrication operations.
As the feature sizes of microelectronic integrated circuits devices are reduced, the electrical properties of the materials that constitute the devices will require change and improvement. One material that must be improved is the electrical insulator (“dielectric”) used between the wires, metal lines, and other elements of the circuit. Without improvement in the insulator material, there will be increased problems due to capacitive effects such as coupling (crosstalk) and propagation delay. The speed at which future circuits will operate will be limited by RC delay in the interconnect. These difficulties can be mitigated by preparing the circuit using an insulating layer that possesses a dielectric constant as low as possible (i.e., a “low-k” dielectric layer).
Earlier technology nodes (i.e., the sets of VLSI fabrication technologies associated with particular critical dimensions) employed dense materials such as silicon dioxide, silicon nitride, and cured silsesquioxanes as insulators. However, the dielectric constants of these materials range from 3.0-7.0. These values will be inadequate for future circuits. As yet the only fully dense (non-porous) materials with a dielectric constant less than about 2.4 are fluorinated polymers or fully aliphatic hydrocarbon polymers, but these have not met requirements for adhesion and thermal stability.
Thus, considerable effort has been directed towards the development of porous dielectric materials. These can be thought of as composite materials, with the value of their dielectric constants intermediate between that of air (dielectric constant of 1.0) and the fully dense phase. Several classes of porous dielectric films, including porous oxides and porous polymers have been described in the art. Pores in these materials typically have dimensions in the nanometer range (e.g., about 0.1 to 50 nanometers). The materials are sometimes referred to as “nanoporous” or “mesoporous.”
Nanoporous silica films are formed by a variety of techniques. In one example, a process deposits a monomeric precursor such as tetraethylorthosilicate or TEOS (a siloxane) onto a substrate using a solvent, and then cross-links the precursors to form a continuous porous solid network. The resultant films are dried by direct solvent evaporation or treatment with supercritical fluids. The films are then subjected to a high-temperature annealing step. See for example, Changming et al., Materials Research Society Bulletin, vol. 22, no. 10, pp. 39-42 (1997).
Other approaches involve forming a stable composite film (sometimes referred to herein as a “precursor film”) containing two components: a removable porogen and a structure former or dielectric material (e.g., a silicon containing material). After the composite film is formed on the substrate, the porogen component is removed, leaving a structurally intact porous dielectric matrix. Porogens are frequently organic compounds such as polyfunctional cyclic hydrocarbon compounds. Techniques for removing porogens from the composite film include, for example, thermal processes, e-beam exposure, ultra-violet radiation exposure, plasma treatments, and solvent extraction employing supercritical solutions.
In a related approach, a mesoporous dielectric matrix is formed using a block copolymer such as a polyethylene oxide (PEO)-polypropylene oxide (PPO) block copolymer. The polymer is deposited and the PEO and PPO blocks segregate into separate phases, with the PPO phase regions being more hydrophobic and the PEO phase regions being more hydrophilic. The two-phase polymer is sometimes referred to as a “template.” The dielectric matrix is created by infusing a dielectric precursor (e.g., TEOS) and a catalyst into the polymer template where one or both of them preferentially accumulate in one phase. In the favored phase, the catalyst promotes reaction of the dielectric precursor to form the dielectric. Thus, the dielectric matrix forms selectively in one phase of the template. The other phase (the “porogen”) can be removed subsequently by various techniques such as plasma treatment.
As used herein, the term porogen is intended to encompass any type of material that forms a removable second phase in a composite layer that serves as a precursor to the dielectric matrix. The porogen may be arranged in an ordered or non-ordered fashion within the dielectric matrix.
Another technique for producing porous low-k films employs inorganic-organic nanophase-separated hybrid polymer materials. These materials comprise organic polymers cast with silsesquioxane-based ladder-type polymeric structures. A casting solvent is used to dissolve the inorganic and organic polymer components. The materials are spin coated onto a substrate and upon application of high-temperature, the hybrid phase-separated polymer materials are formed. See for example, Miller et al., Materials Research Society Bulletin, vol. 22, no. 10, pp. 44-48 (1997).
Certain problems arise in VLSI processing of partially fabricated devices having a porous dielectric material. A first problem arises in porous “open cell” dielectric materials, in which the individual pores contact and open into one another. The pores of these materials provide long paths throughout the interior of the dielectric material. Gases and liquids contacting the outer surfaces of open cell dielectric materials can penetrate deep into the layer's interior. This gives rise to a particularly difficult problem during conformal depositions of conductive barrier layers or seed layers. Precursor gases or plasma for these processes penetrate deep into the open cell matrix where they deposit and form the conductive barrier or seed materials. This renders large portions of the dielectric layer unacceptably conductive. Problems can also arise during etch and strip processes in which the gases or plasma penetrating the pores do not result in deposition but degrade the dielectric material or increase its dielectric constant. A similar problem is now encountered in closed cell dielectric materials, even where the pore sizes are significantly smaller than the VLSI feature dimensions.
Examples of extremely conformal deposition processes where the problem is most pronounced include certain forms of chemical vapor deposition (CVD) and atomic layer deposition (ALD). Less conformal processes such as physical vapor deposition (PVD) do not deposit conductive material within the pore network, but they do a poor job of covering the discontinuous porous side-walls of a trench or via.
Another problem arises because porous materials lack the mechanical strength of non-porous materials. As a consequence, when a planarization technique such as chemical mechanical polishing (CMP) is employed to remove excess copper or other material, the pressure applied to the wafer during that process can crack or crush the underlying dielectric material.
The current porous materials and associated processing techniques have thus far failed to meet the demands of next generation VSLI fabrication. Obviously, the problems will only increase as technologies move to smaller feature dimensions. Improved methodologies for integrating low-k dielectric materials into VLSI devices are required.